The University of Washington team believe it may be possible to develop a drug to tackle the problem.

Researcher Dr Horacio de la Iglesia said: "If we can discover how the two parts of the brain are synchronised we might be able to find mechanisms to treat jet lag."

A tiny area of the brain called the suprachiasmatic nucleus (SCN) is thought to control sleep, hormone and temperature rhythms in the body.

It is this area, it is thought, which responds to external factors such as sunrise.

However, other characteristics stick more closely to regimented 24-hour cycles - suggesting the presence of another timekeeping centre - possibly within the same tissue - that is unaffected by external cues.

Night and day mixed

The researchers examined rats exposed to artificial days and nights lasting 11 hours, instead of the usual 12.

As the experiment went on, they found that the rats gradually began to show signs of normal daytime behaviour at night.

The researchers examined levels of two proteins in the animals' brains. One, Perl, is usually present during the day, the other, Bmall, is usually present at night.

When the rats behaved normally, their SCNs contained Perl when the lights were on - simulating daytime - and Bmall when they were off.

However, when the animals started to exhibit "daytime" behaviour at night, the researchers found evidence of the presence of both proteins in the SCN at the same time - Perl was found in the top half and Bmall in the bottom.

This suggests that the bottom of the brain's timekeeping centre can follow a 24-hour cycle as it continued to produce the "right" protein when the lights were out.

But is also suggests that the top half is more likely to respond to external cues.

The researchers believe that jet lag occurs when the two halves become desynchronised.

Dr Mary Morrell, a lecturer in sleep physiology at the National Heart and Lung Institute in London, told BBC News Online: "It is probable that there is more than one timekeeping centre in the brain, but what is not clear is how they are linked together."

Dr Morrell said many biological systems worked on the two centre principle - a primary centre, and a secondary centre for fine-tuning.

She said many current psychoactive treatments had a broad-based effect. The key to the successful development of more effective treatments would be to develop drugs that had a more specific effect on the brain.